The present invention relates to a process and apparatus for injection molding thin walled articles such as hollow containers.
Molded articles such as thin walled containers have traditionally been produced by injection molding, usually with a hot runner mold directly gated into the bottom of the container. Limitations on molding cycle and minimizing the sidewall thickness are often imposed by the thickness and configuration of the article's lip. In some container styles the lip is considerably thicker than the side wall, typically a lip 0.30-0.040" compared to a sidewall thickness of 0.015-0.020". It becomes increasingly difficult to properly pack the lip and to eliminate sink marks, when the resin must be forced through the restriction of a very thin sidewall from a gate at the bottom of the part. Generally, the resin must be raised higher in temperature to promote flow through the thin sections. The consequences of doing this are that the molding cycle is slowed by the time taken to cool the thickest part of the container--the lip.
One approach which has been found to overcome many of the molding problems associated with molding thick lip/thin sidewall articles is injection blow molding. U.S. Pat. No. 4,540,543 to Thomas illustrates a typical blow molding system for forming thin walled containers. There are however several problems which remain.
Injection blow molding requires specialized molds and expensive support equipment. Further there is the problem of the tendency to create a mold separation force when injecting the resin into the bottom of the part. This occurs because the top of the mold core acts as a post. As the resin flows under pressure against the top of the mold core, the mold core is pushed away from the mold cavity. The mold separation force thus created can be quite large because the relative cross section mold area at the bottom of the part is relatively large.
Another approach which has been used to form thin walled articles is sequential injection molding. In this approach, a first plastic material is injected into a cavity through a first gate. This material is then cooled to solidify the first plastic. Subsequent to injecting the first plastic, a second plastic material is injected through a second gate into the cavity. The second plastic fills the cavity and fuses with the previously injected plastic. Thereafter, the second plastic is cooled to solidify the fused unit. U.S. Pat. Nos. 4,381,275 and 4,508,676, both to Sorenson, illustrate this approach.
The purpose of sequential injection is to have the cooled first plastic material stabilize the core section by impeding movement of the core section caused by injection of the second plastic material. This approach however has not been entirely successful in eliminating the side load problem. Additionally, it does lead to the formation of weak weld lines where the first and second plastic materials are fused. Weld lines are typically at their strongest when both melt edges are at the same high temperature.
Another method for forming molded articles having comparatively thick lip portions is shown in U.S. Pat. No. 4,622,002 to Bormuth. A split cavity mold is shown with an edge gating system feeding directly into the lip of the part. The primary disadvantage of this arrangement is that the alignment of the cavity halves with the core will not adequately resist the considerable side load induced by the injection of the resin at the lip. This side force will cause the mold core to shift out of axial alignment with the mold cavity before the resin has filled the cavity. This in turn will result in a part having an unequal sidewall thickness. A second disadvantage is that the mold must contain complicated mechanisms to synchronize the opening and closing of the cavity halves during the cycle and the runner system must be arranged so the runner axis is perpendicular to the core, and the axis of the machine. This entails additional complexity and expense. A third disadvantage is the formation of gate vestiges on the outer perimeter of the lip.
Parent application Ser. No. 221,026 describes a molding approach which overcomes these problems. In this approach, the plastic material is directly gated into the lip of the thin wall article without sacrificing core/cavity alignment and conventional mold motions along the machine axis. By filling the comparatively thick lip or rim portion first and then having the plastic material flow through the sidewall portions, substantially equal forces act against the sides of the core and the cavity.
While this direct gating approach has been quite useful, there is still room for improvement. Molding approaches which reduce the pressures acting in the space defined by the core/cavity arrangement, which operate at faster speeds, and which can be used to form very thin parts are always being sought.
Accordingly, it is an object of the present invention to provide an injection molding process and apparatus for forming thin walled articles.
It is a further object of the present invention to provide a process and apparatus as above which allows articles to be formed with less pressure.
It is yet a further object of the present invention to provide a process and apparatus as above which allows articles to be formed in a faster time.
It is still another object of the present invention to provide a process and apparatus as above which does not create any significant side loads so as to cause misalignment of the mold cavity/core portions.
These and other objects and advantages will become more apparent from the following description and drawings wherein like reference numerals depict like elements.